Collectively, the independent risk factors of overweight and obesity, physical inactivity, and inadequate diet are second only to tobacco as modifiable risk factors for cancer.[1][2] Healthy eating and physical activity protect against certain cancers both directly, and indirectly, through their impact on maintaining a healthy body weight.

Addressing these risk factors is an important objective for reducing Australia’s cancer burden given obesity and overweight have reached record levels in the Australian population. Nearly two in three (63.4%) Australians aged 18 years and over are overweight or obese[3] and one in four (27.4%) Australian children are overweight or obese.[3]

The World Cancer Research Fund (WCRF) systematically reviews global evidence on diet, weight, and physical activity in relation to cancer risk according to levels of evidence. There is convincing evidence that overweight and obesity increase the risk of cancers of the oesophagus, pancreas, bowel, breast (post-menopausal), endometrium, kidney and liver.[4] Furthermore, there is probable evidence that overweight and obesity increase the risk of cancers of mouth, pharynx, larynx, ovary, prostate, gallbladder and stomach.[4] Physical activity has been shown to convincingly protect against bowel cancer and probably protect against breast (post-menopausal) and endometrium cancers.[4] Dietary factors probably protect against cancers of the bowel.[4] Further to this, the International Agency for Research on Cancer (IARC) has shown there is sufficient evidence that high body mass increases the risk of 13 types of cancer.[5]

In 2011, the Australian Institute for Health and Welfare found 6.4% of total cancer burden (number of years of life lost due to cancer) was attributable to physical inactivity followed by high body mass (4.5% of total burden).[6] In 2015, Cancer Council commissioned a study to quantify the numbers of cancer cases attributable to modifiable factors in Australia. The study found that 3.4% of cancers in 2010 were attributable to overweight/obesity[7] and 1.6% of cancers were due to insufficient physical activity.[8] Dietary factors were attributable for 6.1% of cancers with inadequate fibre intake accounting for 18% of bowel[9] and red and processed meat consumption accounting for a further 18% of bowel cancers diagnosed in Australians in 2010.[10] For further information on the link between tobacco smoke and cancer risk (not covered in this chapter), see the Tobacco control chapter of the National Cancer Control Policy.

Table 1 shows the estimated number of cancers diagnosed in Australia in 2010 attributable to these modifiable risk factors. The separate estimates for each risk factor cannot be added or combined due to the complex relationships between them.

In 2017, a new estimate of numbers of cancer deaths attributable to modifiable factors in Australia in 2013 was published. Dietary factors were found to be the second highest cause of cancer deaths (2,329 deaths) followed by overweight/obesity (1,990 deaths).[11]

As well as being a direct cause of several cancers[4][12], alcohol consumption may indirectly increase cancer risk by contributing to obesity and overweight. For further information on the link between alcohol consumption and cancer risk (not covered in this chapter), see the Alcohol chapter of the National Cancer Control Policy.

This chapter of Cancer Council Australia’s National Cancer Control Policy discusses: the public policy context and effective policy interventions that can reduce the prevalence of these cancer risk factors. The chapter also summarises Cancer Council Australia’s recommended, evidence-based policy priorities for reducing the cancer burden in Australia attributable to obesity and overweight, physical inactivity and inadequate diet. The impact in Australia of cancers related to obesity and overweight, physical inactivity, and inadequate diet and evidence on the links between these factors and cancer are also covered.